The present invention relates to an electroluminescent device (hereinafter referred to as xe2x80x9cEL devicexe2x80x9d) having a luminescent layer which comprises luminescent particles and a binder resin. In particular, the present invention relates to an EL device which can achieve a high luminescent efficiency.
EL devices having a so-called xe2x80x9cdispersion type luminescent layerxe2x80x9d which is formed by dispersing luminescent particles such as phosphor particles in binder resins such as polymers having a high dielectric constant are known from the following publications.
For example, JP-B-59-14878 discloses an EL device comprising a transparent substrate, a transparent electrode layer, an insulating layer consisting of a vinylidene fluoride binder resin, a luminescent layer comprising a vinylidene fluoride binder resin and phosphor particles, the same insulating layer as above, and a rear electrode, which are laminated in this order. JP-B-62-59879 discloses an EL device comprising a polyester film, an ITO electrode, a luminescent layer which comprises phosphor particles and a cyanoethylated ethylene-vinyl alcohol copolymer (a binder resin), and an aluminum foil (a rear electrode), which are laminated in this order.
Unfortunately, however, it is difficult to increase the luminance in the case of such xe2x80x9cdispersion type luminescent layersxe2x80x9d. The reason for this is that luminescent particles, which have a larger specific gravity than binder resins, tend to sink in a coating for forming luminescent layers comprising luminescent particles dispersed in the solution of binder resins, and thus it is difficult to uniformly disperse the luminescent particles in the luminescent layers formed from such a coating. Furthermore, the dispersibility deteriorates when the amount of luminescent particles in the coating is increased to increase the filling rate of luminescent particles in the luminescent layer. The filling rate of the luminescent particles is at most 20 volume % of the coating weight. In addition, it is relatively difficult to increase the coating thickness of the luminescent layer while maintaining the uniformity of a thickness using such a dispersion type coating. Therefore; when the number of applications of the coating is increased to increase the thickness of the luminescent layer, the productivity decreases, and it is difficult to produce a roll-form EL device having a large area.
EL devices having a xe2x80x9clamination type luminescent layerxe2x80x9d are known as one measure to solve the drawbacks of the xe2x80x9cdispersion type luminescent layersxe2x80x9d. For example, U.S. Pat. Nos. 5,019,748 and 5,045,755 disclose an EL device having a lamination type luminescent layer, which consists of a three-layer laminate comprising: (1) a first dielectric adhesive layer with a high dielectric constant applied on the transparent conductive layer of a transparent substrate; (2) a phosphor particle layer in the form of a substantially single layer (having a thickness not exceeding the largest size of particles), which is formed by electrostatically applying dry phosphor particles (luminescent particles) on the first dielectric adhesive layer; and (3) a second dielectric layer placed on the phosphor particle layer and containing a dielectric material with a high dielectric constant, which layer fills the spaces between adjacent phosphor particles. A rear electrode is applied on the surface of the second dielectric layer, and thus the second dielectric layer functions as an insulating layer.
In contrast with the above xe2x80x9cdispersion type luminescent layerxe2x80x9d, it is possible to continuously carry out the coating processes, and it is possible to produce a roll-form EL device by the disclosed method. However, the above publications and patent specifications do not disclose any specific manner to form a continuous terminal (buss), through which electricity (voltage) may be applied from outside to the transparent conductive layer, e.g., along the lengthwise direction of the transparent substrate in the production process of a roll-form EL device.
To increase the area of EL devices, it is a key factor how a terminal (buss), which supplies electricity (voltage) to a transparent conductive layer from the outside, is provided. For example, in the case of EL devices for displays with a small area, busses which are not electrically in contact with a rear electrode, can be formed on a transparent conductive layer by effectively repeating screen printing. However, none of the above cited publications or patents disclose the formation of busses continuously in the lengthwise direction of the device, or any methods for such formation.
U.S. Pat. No. 4,143,297 refers to electroluminescent information display panels which are said to be suitable for uses extending from simple numeric displays to color TV panels. The display panel comprises a body of insulating resin having a layer of electroluminescent particles embedded therein. This layer is a monoparticle layer. The resin has a dielectric constant higher than that of the particles and includes fluorescent material on at least one side of the layer of electroluminescent particles. Furthermore, insulating coatings on both front and back surfaces of the resin body, a transparent front electrode extending over the insulating coating of the front surface, a back electrode disposed on the insulating coating on the back surface and means for electrically energizing the electrons are provided. At least one element of the display panel adjacent the back thereof is black and sufficiently opaque to absorb substantially all the light reaching it.
WO 98/53645 refers to an electroluminescent device and a method for producing the same. Among others the electroluminescent device comprises a luminescent layer comprising a transparent support layer comprising a matrix resin, an insulating layer comprising an insulating material and a luminescent particle layer consisting essentially of particles which comprise luminescent particles and which are embodied in both the support layer and the insulating layer.
Conventional xe2x80x9clamination type luminescent layersxe2x80x9d have several drawbacks. For example, EL devices having xe2x80x9clamination type luminescent layersxe2x80x9d can emit light at a luminance equal to or higher than that of EL devices having xe2x80x9cdispersion type luminescent layersxe2x80x9d when they are connected with a power source having the same frequency and the same voltage. However, the luminescent efficiency is not improved so greatly, or sometimes it may deteriorate.
Luminescent efficiency (xe2x80x9cxcex7xe2x80x9d) is a value defined by the following formula:
xcex7=Lxc3x97xcfx80xc3x97S/P
where:
P is a used electricity (effective electric power) (unit: W),
L is a luminance measured with a luminance meter (unit: cd/m2),
S is the area of a luminescent surface, and
xcfx80 is the ratio of the circumference of a circle to its diameter.
In other words, a low luminescent efficiency means a low luminance per unit effective electric power, and thus a low power efficiency. Accordingly, it is a goal to improve the luminescent efficiency from the viewpoint of energy-saving.
In one embodiment, the present invention provides an EL device having an effectively improved luminescent efficiency. Preferred such electroluminescent devices comprise:
a transparent conductive layer,
a binder layer placed on the back surface of the transparent conductive layer, a luminescent-particle layer comprising a substantially single layer of particles containing luminescent particles, which layer is applied on the back surface of the transparent conductive layer through the binder layer,
an insulating layer comprising insulating particles, which is placed on the back surface of the luminescent-particle layer, and
a rear electrode placed on the back surface of the insulating layer, wherein the luminescent particles are embedded in the binder layer, or the luminescent particles are substantially not embedded in the insulating layer.
In another embodiment, the present invention provides a method for the production of an EL device, which method can produce a sheet-form EL device having a high luminescent efficiency at a high productivity without the use of the above dispersion coating.
Preferred methods for the production of an electroluminescent device (which optionally comprise the features described above) comprise the steps of:
applying a coating for the formation of a first layer of a binder layer on either one of the back surface of a transparent conductive layer and the surface of an insulating layer, placing particles containing luminescent particles in a layer form on the applied coating prior to the solidification of the coating, and solidifying the coating after partly embedding the layer of the particles, to form the first layer of a binder resin and the luminescent-particle layer adhered to the first layer,
applying a coating for the formation of a second layer of a binder layer on the luminescent-particle layer, and solidifying the coating, to embed the luminescent particles in the binder layer consisting of the first and second layers without exposing the surfaces of the luminescent particles, and
applying the other of the transparent conductive layer and the insulating layer on the binder layer in which the luminescent particles are embedded.
In yet another embodiment, the present invention provides an EL device which can be produced in a roll-form from which a large-size luminescent device can be easily produced.
In this embodiment, the present invention provides an electroluminescent device as described above, in which the transparent conductive layer, luminescent-particle layer, insulating layer and rear electrode preferably continuously extend along the length of the transparent conductive layer. The device further preferably comprises at least one buss which is electrically in contact with the back surface of the transparent conductive layer, has a width smaller than the width of the transparent conductive layer and continuously extends along the length of the transparent conductive layer, and the buss is not electrically in contact with the rear electrode.
One of the characteristics of the EL device according to one embodiment of the present invention is that luminescent particles are embedded in a binder layer. Thereby, the efficiency of luminance in relation to an effective electric power (luminescent efficiency) can be increased.
Although not intending to be bound by theory, the function of this structure of an EL device may be assumed as follows:
In conventional lamination type EL devices, spaces between phosphor (luminescent) particles are filled with fillers having a very high dielectric constant (e.g. insulating particles, etc.). Thus, a capacitance in the spaces between the phosphor particles increases. Accordingly, a dielectric loss in such spaces increases, and/or an electric power is lost due to the generation of Joule heat. Therefore, the luminescent efficiency decreases.
In general, the dielectric constant of insulating particles is at least 100, and typical insulating materials having a relatively high insulating effect such as barium titanate have a dielectric constant of 1,000 or larger. In contrast with such insulating particles, organic polymers or high dielectric polymers, which can be used as binder resins (sometimes called as xe2x80x9cmatrix resinsxe2x80x9d), usually have a dielectric constant of less than about 50, and preferable high dielectric polymers such as vinylidene fluoride resins and cyanoresins have a dielectric constant of from about 5 to about 30. Herein, a dielectric-constant is a specific dielectric constant measured under the application of an alternating current of 1 kHz, unless otherwise specified.
In the above construction of the present invention, luminescent particles are preferably embedded in a binder resin layer, and preferably few (or more preferably effectively no) insulating particles having a very high dielectric constant are present in spaces between adjacent luminescent particles. Thus, the capacitance in such spaces can be effectively decreased.
One of the characteristics of an EL device according to another embodiment of the present invention is that luminescent particles are substantially not embedded in an insulating layer. When a luminescent-particle layer is substantially not embedded in an insulating layer, fillers having a very high dielectric constant (e.g. insulating particles, etc.) do not fill the spaces between the phosphor (luminescent) particles, like in the above embodiment. Accordingly, it is possible to suppress the increase of a dielectric loss and the electric power loss due to the generation of Joule heat in such spaces as much as possible, and thus a luminescent efficiency can increase. Such a structure can be easily formed, for example, by embedding luminescent particles in a binder layer, like in the above case, so that the particle surfaces do not expose on the back surface of the binder layer which is in contact with the insulating layer.
Characteristics of an EL device in one preferred embodiment of the present invention are that a transparent conductive layer, a luminescent-particle layer, an insulating layer and a rear electrode continuously extend along the lengthwise direction of a transparent electrode layer, and that the device further comprises at least one buss which is electrically in contact with the back surface of the transparent conductive layer and has a width smaller than the width of the transparent conductive layer and continuously extends along the lengthwise direction of the transparent electrode layer. Another preferred characteristic is that the buss is not electrically in contact with the rear electrode. Thus, it is possible to produce a roll-form EL device, from which a large-sized luminescent display can be easily formed.
When a buss is not in direct contact with a luminescent layer, it becomes more easy to form a roll-form EL device having a large area, since a rear electrode can be applied onto substantially the whole back surface of the luminescent layer, and thus substantially the whole surface of the luminescent layer can emit light.
For example, a buss can be in direct contact with the edge area of a luminescent layer. However, in this case, the buss and an electrode-free area in which no rear electrode is applied should be provided on the back surface of the luminescent layer to separate the rear electrode and the buss, so that the buss and rear electrode are not electrically in contact with each other. A part of the light-emitting surface of the luminescent layer, which corresponds to the electrode-free area, can emit substantially no light, and thus the light-emitting area may not be increased.
The EL device of the present invention can be produced by various methods. For example, it is preferably produced by a method, which comprises the steps of:
applying a coating for the formation of the first layer of a binder layer on either one of the back surface of the transparent conductive layer and the surface of the insulating layer, placing particles containing luminescent particles in a layer form on the applied coating prior to the solidification of the coating, and solidifying the coating after partly embedding the layer of the particles, to form the first layer of a binder resin and the luminescent-particle layer adhered to the first layer,
applying a coating for the formation of the second layer of a binder layer on the luminescent-particle layer, and solidifying the coating, to embed the luminescent particles in the binder layer consisting of the first and second layers without exposing the surfaces of the luminescent particles, and
applying the other of the transparent conductive layer and the insulating layer on the binder layer in which the luminescent particles are embedded.
The above method can produce an EL device having an improved luminescent efficiency at a good productivity. Furthermore, a sheet-form EL device having a large area or a roll-form EL device can be easily produced.